This invention relates to diesel fuel injectors of the mechanical port-closing and spill type as used in various models of EMD locomotive, marine and power generation engines, and to components and means related to such injectors. These injectors are often referred to a xe2x80x9cunit injectorsxe2x80x9d because both injection pump and nozzle are combined in a single unit.
Mechanical injectors used in EMD engines are characterized by a fuel delivery system comprising, together with other parts, a plunger and bushing assembly having two ports in the bushing side wall, axially and diametrically spaced from each other. When the upper or xe2x80x9cfillxe2x80x9d port is completely covered or xe2x80x9ccut offxe2x80x9d by an upper control edge of the plunger during the down stroke of the plunger, delivery of high-pressure fuel to the nozzle begins. When, during the continued downward movement of the plunger, the lower or xe2x80x9cspillxe2x80x9d port starts to be uncovered by the plunger""s lower control edge, fuel is spilled to low-pressure areas of the system and thereby drops to fuel supply pressure, and fuel delivery stops.
Mechanical injectors presently used in EMD engines employ both variable start of injection and variable end of injection relative to engine top dead center over the operating load range of the engine. The reason for using this timing and fuel control relationship dates back many years to when locomotive engine injectors were designed to reduce objectionable engine knocking at part load and idle operation. Knocking was particularly objectionable when locomotives idled in railroad stations. To address the problem, the upper control edge of the injector""s pump plunger was given a constant helix angle so as to increase the degree of retardation as a straight line, inverse function of engine load as the latter decreased from full load all the way down to idle. The helix angle was relatively high, and correspondingly the increase in retardation over the range was also relatively high.
A disadvantage of this arrangement is high levels of nitrous oxide (NOx) production when the engine operates at full load or at close to full load. Another disadvantage is higher than normal fuel usage when idling or operating at part load because of the relatively high retardation of the start of injection at such load settings. Nevertheless, in the context of the prevailing engine requirements of many years ago, the arrangement performed quite well. At that time exhaust emissions, except for smoke, were not a concern and were not considered in optimizing the overall performance of the engine.
With today""s environmental concerns, exhaust emissions become a dominant factor. Oxides of nitrogen are today the emissions constituent of greatest concern, and many means have been devised to reduce it to meet EPA""s most stringent requirements, while at the same time maintaining acceptable fuel economy. These improvements involve complex design modifications such as electronic control of injection timing and fuel output, pilot injection, or injection rate shaping. All these improvements produce quite beneficial emissions reductions, but they require replacement of complete injectors with costly injector modifications.
The present invention provides simple relatively low cost means to reduce oxides of nitrogen in the higher load operating range of the engine, and to achieve compliance with current emissions standards in respect of both NOx and particulates when emissions at the various load points are properly rated as prescribed by current EPA regulations, while at the same time providing acceptable fuel efficiency.
In an article-of-manufacture aspect, the invention involves plungers having one or more steps formed in their upper (cut-off) control edge between adjacent segments of the edge that are associated with adjacent load positions and are of 0 degrees helix angle, as more fully described below. Also, the upper port-closing control edge may be shaped to retard start of injection at full power from what it is with standard plungers; it may also be shaped to provide a smaller increase in retardation from full load to idle as compared to standard plungers, and to do so in a non-straight-line manner.
In a method aspect, the invention involves the concept of replacing the standard plungers of EMD-type injectors with prototype plungers shaped as described in the preceding paragraph, then assessing omissions/fuel-economy performance associated with the prototype plungers over all load points when operating in otherwise standard EMD injectors, then selecting a new pattern of timings as indicated by the assessment, making new prototype plungers and assessing them, repeating the steps of selecting, making and assessing if and as required until an assessment indicates compliance of the newest prototype plunger with applicable performance standards, and thereafter using replacement plungers having the form of the complying prototype in the standard injectors.
The prior art does include plungers, intended for use in EMD-type injectors, whose upper control edges are shaped such that start of injection is retarded at full power from what it is with standard plungers, but such plungers do not provide any change in retardation from full load to idle and have a poor overall emissions performance when used as substitutes for standard plungers in EMD type injectors.